Bad money drives good money out of circulation

When a government issues two coins with the same face value but different amounts of precious metal, people quietly hoard the coin with more silver and spend the debased one as fast as possible. The good coins vanish from everyday circulation, saved rather than spent, while the bad ones do all the work of money. Named for a 16th-century financier's advice to Elizabeth I, the pattern still shows up whenever a currency is quietly devalued.

— Henry Dunning Macleod, The Elements of Political Economy — 1858 — coined the term 'Gresham's law'